Method and apparatus for multiwavelength imaging spectrometer

ABSTRACT

The disclosure generally relates to a method and apparatus for multi-wavelength imaging spectrometer. More specifically, in one embodiment, the disclosure relates to an optical filter for passing photons therethrough. The filter includes a first filter stage and a second filter stage. The first filter stage may include a first retarder element and a first liquid crystal cell. The first element may include an input face and an output face. One of the first element faces is not oriented substantially normal to the trajectory of photons passing through the filter.

RELATED APPLICATIONS

The instant application claims the filing date priority of U.S. Provisional Application No. 60/488,246 filed Jul. 18, 2003, the specification of which is incorporated herein in its entirety. Cross-reference is made to patent applications filed simultaneously herewith and entitled Method and Apparatus for Compact Birefringent Interference Imaging Spectrometer (Attorney Docket No. CHE01 009), Method and Apparatus for Compact Dispersive Imaging Spectrometer (Attorney Docket No. CHE01 010) and Method and Apparatus for Compact Resonant Cavity Imaging Spectrometer (Attorney Docket No. CHE01 012), the specification of each of the cross-referenced applications being incorporated herein in its entirety.

BACKGROUND

Spectroscopic imaging combines digital imaging and molecular spectroscopy techniques, which can include Raman scattering, fluorescence, photoluminescence, ultraviolet, visible and infrared absorption spectroscopies. When applied to the chemical analysis of materials, spectroscopic imaging is commonly referred to as chemical imaging. Instruments for performing spectroscopic (i.e. chemical) imaging typically comprise image gathering optics, focal plane array imaging detectors and imaging spectrometers.

In general, the sample size determines the choice of image gathering optic. For example, a microscope is typically employed for the analysis of sub micron to millimeter spatial dimension samples. For larger objects, in the range of millimeter to meter dimensions, macro lens optics are appropriate. For samples located within relatively inaccessible environments, flexible fiberscopes or rigid borescopes can be employed. For very large scale objects, such as planetary objects, telescopes are appropriate image gathering optics.

For detection of images formed by the various optical systems, two-dimensional, imaging focal plane array (FPA) detectors are typically employed. The choice of FPA detector is governed by the spectroscopic technique employed to characterize the sample of interest. For example, silicon (Si) charge-coupled device (CCD) detectors or CMOS detectors are typically employed with visible wavelength fluorescence and Raman spectroscopic imaging systems, while indium gallium arsenide (InGaAs) FPA detectors are typically employed with near-infrared spectroscopic imaging systems.

A variety of imaging spectrometers have been devised for spectroscopic imaging systems. Examples include, without limitation, grating spectrometers, filter wheels, Sagnac interferometers, Michelson interferometers and tunable filters such as acousto-optic tunable filters (AOTFs) and liquid crystal tunable filters (LCTFs).

A number of imaging spectrometers, including acousto-optical tunable filters (AOTF) and liquid crystal tunable filters (LCTF) are polarization sensitive, passing one linear polarization and rejecting the orthogonal linear polarization. AOTFs are solid-state birefringent crystals that provide an electronically tunable spectral notch pass band in response to an applied acoustic field. LCTFs also provide a notch pass band that can be controlled by incorporating liquid crystal retarders within a birefringent interference filter such as a Lyot filter. Conventional systems are generally bulky and not portable. A handheld chemical imaging sensor capable of performing instant chemical analysis would represent progress in size, weight and cost reduction. Accordingly, there is a need for a handheld, portable and more efficient tunable filter.

SUMMARY OF THE DISCLOSURE

In one embodiment, the disclosure relates to an optical filter for passing photons therethrough. The filter includes a first filter stage and a second filter stage. The first filter stage may include a first retarder element and a first liquid crystal cell. The first element may include an input face and an output face. One of the first element faces is not oriented substantially normal to the trajectory of photons passing through the filter.

In another embodiment, the disclosure relates to a an optical filter having a first liquid crystal cell with a first region having a first photon retardation value and a second region with a second photon retardation value. The optical filter can also include a second liquid crystal cell with a first region having said first photon retardation value and a second region having said second photon retardation value. Each of the first and the second retardation values can be defined by a first excitation source and a second excitation source communicating respectively with each of the first and the second regions of said first and the second cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a conventional line scan Raman imaging system;

FIG. 2 is a schematic illustration of a conventional wide-field scan Raman imaging system;

FIG. 3 is a schematic representation of a three-stage conventional Lyot liquid crystal tunable filter;

FIG. 4A schematically illustrates a uniform liquid crystal cell of an electronically controlled birefringence cell;

FIG. 4B schematically illustrates a wedged liquid crystal cell of an electronically controlled birefringence cell;

FIG. 4C is a schematic illustration of an optical device and an optical stage according to one embodiment of the disclosure;

FIG. 5 is a schematic illustration of an optical stage according to another embodiment of the disclosure;

FIG. 6 schematically illustrates a stepped LCTF device for un-patterned Raman imaging or other chemical imaging applications according to another embodiment of the disclosure;

FIG. 7 is a schematic illustration of a columnized LCTF device for Raman imaging or other chemical imaging applications according to another embodiment of the disclosure;

FIG. 8 is a schematic illustration of a columnized LCTF device for Raman or other chemical imaging applications according to one embodiment of the disclosure;

FIG. 9 is a schematic illustration of an un-patterned Raman imaging or other chemical imaging applications system according to another embodiment of the disclosure;

FIG. 10 is a schematic illustration of imaging system using tunable Fabry-Perot;

FIGS. 11A-11C are schematic illustrations of a handheld Chemical Imaging Threat Assessor detection device (CHITA) according to one embodiment of the invention;

FIG. 12 is a secondary illumination source according to one embodiment of the disclosure;

FIG. 13 schematically illustrates the operation of an exemplary detection device;

FIG. 14 provides exemplary packaging options according to one embodiment of the disclosure; and

FIG. 15 shows dispersive Raman spectroscopy of a sample through PMMA.

DETAILED DESCRIPTION

FIG. 1 is a schematic representation of one type of conventional line scan Raman imaging system, in particular a dispersive scan Raman imaging system employing a “pushbroom” scanning arrangement. A sample 105 is illuminated from a source 110, and light energy is reflectively back scattered from the sample and collected at optics 115. The image of the sample is accumulated from successive images at adjacent parallel lines in the image. The image of the sample is obtained for each such line and also at each of a plurality of specific wavelengths in the spectrum. Thus, a line or one-dimensional set of light amplitude values is sampled at a given location X₁, Y₁ . . . Yn. The light is filtered for specific wavelengths using a spectrometer 120 that functions as a variable wavelength bandpass filter. By advancing from line to line in a “pushbroom” sequence, the successive amplitude values shown graphically at plot 140 are converted to intensity pixel values in an array 160 for a given wavelength λ₁. By sampling at different wavelengths, a separate substantially monochromatic image 160 is collected for each wavelength value λ₁ to λ_(n). An object of this technique conventionally is to produce a set of monochromatic images that can be compared to one another to assist in identifying features in the image that may be characterized by contrast in their intensity versus adjacent features and other wavelengths.

The source 110 may be a laser, a fluorescent source or another source. Reflectively scattered photons are received by optical objective 107 and directed to spectrometer 120 via optical device 115. The particular arrangements for collecting the image of a line of pixels can involve focusing an image on a linear photosensor array of CCDs or the like, or scanning and sampling with one photosensor. The spectrometer 120 needs to be tunable in some way to selectively pass one wavelength bandpass at a time. It is possible in different configurations to use gratings or prisms or birefringent crystal arrangements for wavelength selection. In tuning from on wavelength to another and in advancing from one line to another (in whatever order), it may take a substantial time to collect the amplitude values for each pixel position and each wavelength.

FIG. 2 is a schematic illustration of a conventional two dimensional Raman imaging system. This system operates somewhat the same as the of FIG. 1, namely illuminating sample 205 using source 210, collecting the reflected image via optics 215, selectively passing a bandpass wavelength via a tunable spectrometer 220 and sampling the image at a photosensitive array 230. However this embodiment collects a two dimensional array of pixel amplitudes during each sampling period.

The spectrometer 220 is effectively an imaging or two dimensional tunable wavelength bandpass filter. By repeatedly sampling and tuning to one wavelength or color and then another, any number of wavelength specific images of the sample can be collected and compared. The spectrometer or tunable filter can be arranged to select one or more specific bandpass wavelengths or to reject specific wavelengths. The passes two pixel dimension spectral image 225 can be collected by a CCD camera 230 to produce several spectral images 240 that can be sampled as pixel data images 260 for each wavelength.

An advantageous tunable wavelength bandpass filter can comprise birefringent crystals and polarizers that are variably spaced and/or rotationally adjusted to select for particular wavelengths and to reject other wavelengths. A liquid crystal tunable filter (LCTF) is an advantageous device for this purpose because its birefringence can be tuned electrically. The liquid crystal filter may be nematic or smectic.

Birefringent materials have different indices of refraction for light energy that is polarized along two orthogonal axes, sometimes known as the fast and slow axes. This has the effect of retarding light polarized along one axis compared to light polarized along the other axis. The extent of retardation depends on factors including the indices of refraction of the material and the thickness of the material along the path through which the light is passed. The difference in the propagation time for light polarized on one axis versus the other axis is a time difference. When considered for different light wavelengths, a given time difference equates to a phase angle difference for any given wavelength, but the phase angle difference is a different angle for two different wavelengths.

Retardation of light polarized on one axis more than light polarized on the other axis can have the effect of changing or re-aligning the polarization state of the light. The extent of re-alignment likewise differs with wavelength. For these reasons, polarization and birefringence are useful considerations for a wavelength bandpass filter.

The polarization state of the light that enters or exits a birefringent crystal may be selectively controlled. If a plane polarizing filter (or “polarizer”) is placed to filter randomly polarized light, for example at the input side of the crystal, the passing light can be limited to light energy that is aligned more or less to one or the other of the birefringence axes. If light is aligned to one axis, then rotating the polarizer by 90 degrees aligns the polarizer exclusively to the other birefringence axis. Polarization filters can be used on the input and output sides of a birefringent crystal, to select the nature of the input signal applied to the crystal and to selectively pass only so much of the output as is aligned to the output polarizer.

Assuming that light is initially polarized to a given orientation angle, for example by an input polarizer, then orienting a birefringent crystal at 45 degrees to the orientation angle of the polarizer divides the polarized light into equal vector components, one being aligned to each of the fast and slow axes of the crystal. The retardation of the component on the slow axis relative to the component on the fast axis then changes the polarization state of the light by a rotation angle that depends on wavelength. If an output polarizer is aligned at an appropriate angle for a given wavelength, then that wavelength is transmitted whereas other wavelengths are not.

By manipulation of the alignment of polarizers and birefringent crystals, one can control the allocation of light energy to vector components aligned to the fast axis and the slow axis of the birefringent crystal at each stage. The birefringence of the crystal at each stage retards one of two vector components relative to the other, inducing a phase difference between components on the fast and slow axes of that crystal. The phase retardation between the two components corresponds to a change in the polarization alignment of the light signal, i.e., a change in the angle at which the vector sum of the two components is aligned relative to a reference angle. This change in the polarization state or vector sum angle is specific to wavelength.

There are different ways in which these aspects of light and polarizers and birefringent crystals can be employed as wavelength bandpass filters. In an arrangement including polarizers, a particular wavelength that is passed through a birefringent crystal may be subjected to a polarization change of a specific rotational angle. Other wavelengths are rotated in polarization state by different amounts. By placing a polarizer aligned at that angle on the output of the birefringent crystal stage, only a specific wavelength is passed. Successive stages improve the resolution of the filter.

Some types of wavelength specific filters that can use liquid crystal tunable elements include the Lyot, Solc, Evans and Fabry-Perot wavelength filter configurations, as well as hybrids that employ combinations of the associated elements or additional elements. An LCTF Raman imaging systems using a Lyot filter design for selecting a wavelength spectrum 225 is shown in FIG. 3. The Lyot configuration is an example, and it should be appreciated that the invention is applicable to other LCTF configurations, including but not limited to those mentioned above.

Further, a controller for controlling LCTF detector and shutter as well as a CPU, display unit, key pad and software are conventionally used with the system of FIG. 3. The time for collecting a CCD image and the time for tuning the wavelength bandpass between images both contribute to the time needed to collect a number of wavelength specific images. The image from the detector can be a 2-D image (X,Y) at a given selected wavelength (λ). The LCTF wide-field is capable of producing a 512×512 pixel image, for example.

As stated, conventional tunable filters can use a Lyot filter. A typical Lyot filter, shown in FIG. 3, includes a set of birefringent crystals between two polarizers placed at 45° to the optical axis of the birefringent material. The bandpass wavelength is a function of the thickness of the crystals, among other factors. Entering light from the polarizer is divided evenly between ordinary and extraordinary polarizations by the 45° alignment. The polarizations propagate with different phase velocities due to the birefringence. This alters the polarization alignment of the light energy as a function of wavelength. For a given thickness of birefringent crystal, only one wavelength (or set of spaced wavelengths) is aligned to pass through an exit polarizer. Thus, the filter produces a comb of frequencies. The bandpass wavelength and separation between the “teeth” of the comb depend on the length of the birefringent crystal. In a liquid crystal, the effective optical length is tunably changeable for the polarization component aligned to the extraordinary axis. In this way the bandpass wavelength can be tuned.

A Lyot filter employs multiple birefringent crystals of different lengths, specifically R, 2R, 4R, etc., with polarizers between each crystal. A Solc filter uses equal crystal thicknesses, input and output polarizers only (no interleaved polarizers) and a relative angular orientation between crystals that divides the relative orientation between the input and output polarizers equally among the crystals.

In FIG. 3, sample 315 which can have a multiple wavelengths Raman image receives emitted photons 310 to form scattered photons. The photons scattered by the sample enter Lyot filter 300 which includes four polarizers 320 and three birefringent optical elements 330 defining successive stages. At each stage, an entry-side polarizer 320 acts as an optical filter to pass light at a polarization orientation that is aligned to the polarizer and to block light at the orthogonal orientation. The subsequent birefringent element 330 is oriented at an angle to the preceding polarizer 320, particularly at 45°. Thus, equal vector components of the light passed through the polarizer are aligned to each of the ordinary and extraordinary axes of the birefringent element 330. Orientation of the optical axes 320 for an exemplary Lyot configuration of wavelength bandpass filter is shown in FIG. 3. Other configurations are also known.

The polarization components aligned to the ordinary and extraordinary axes of the birefringent elements 330 propagate with different phase velocities due to the birefringence of elements 330. Also, the birefringent elements 330 at each stage are of a different thickness. By retarding orthogonal components of the light signal, the polarization orientation of the light is realigned to an angle the depends on the wavelength of the light. At the next polarizer encountered, light only at one set of wavelengths is aligned to pass through the next polarizer, which functions as the output polarizer or selector for the preceding stage and as the input polarizer for the next stage. the thickness of the respective birefringent elements 320 and the alignments of the birefringent elements are chosen so that each stage further discriminates for light at the same bandpass wavelength.

The bandpass wavelength is tunable by applying control voltages 335 to the birefringent elements 330, which preferably comprise liquid crystals. The effect of changing the birefringence of the liquid crystals is to shorten or lengthen the effective optical path encountered by the component of the light aligned to the extraordinary axis while leaving the effective length unchanged for the ordinary axis. This is much the same as controllably adjusting the effective thickness of the birefringent elements 330. Each birefringent liquid crystal element 330 of the Lyot filter is coupled to a voltage source 335 for tuning the bandpass of the birefringent elements 330. In a Lyot configuration, the thicknesses are integer multiples (R, 2R, 4R, etc.) and can be controlled in a coordinated manner, for example being coupled to the same control voltage source 335, so as to keep the effective thicknesses equal to the required multiple. In other similar configurations the thicknesses can be otherwise related (for example as in a Solc configuration wherein the thicknesses are equal) and controlled so as to maintain the required relationship, such as to be coupled in parallel to the same driving voltage source.

It is conventional in multispectral imaging to collect individual images in which the entire image is collected at one wavelength bandpass, for comparison with other images at different wavelength band passes. According to one embodiment of the present invention, a novel tunable filter is arranged to tune to different wavelengths across the X-Y image field. The filter may have one or more stages with at least one stage having a wedge-shaped liquid crystal cell or other shape in a tunable configuration having a bandpass wavelength that is not uniform across the surface of the filter, examples being shown in FIGS. 4B, 4C, 6, 7, etc. In these configurations, a wedge shaped or similarly structured birefringent element has a thickness that differs across its operative area. The wedge shaped birefringent element can be tunable and optionally is associated with a uniform (non-tunable retarder). Optionally, a non-tunable wedge shaped birefringent retarder can be combined with a tunable birefringent element of uniform thickness. By combinations of controllable-birefringence and fixed-birefringence elements in uniform an varying thickness along the optical path, different positions in the field are tuned to different bandpass wavelengths.

FIG. 4A schematically shows the elements of a liquid crystal cell of a uniform thickness and electronically controllable birefringence. The uniform thickness liquid crystal shown in FIG. 4A can form a subunit or cell of a liquid crystal filter having plural elements that are stacked along an optical path or arranged adjacent to one another in a pixilized configuration. Each subunit has an associated transparent plate 450, for example of fused silica or glass. On the side of plate 450 that faces liquid crystal cell 465, glass plate 450 has a conductive coating (not shown) such as indium tin oxide (ITO), which is nearly transparent but conducts sufficiently to apply an electric field to liquid crystal material 465 in the element. The conductive surfaces are coupled to leads that provide a driving voltage for each subunit by operation of the LCTF controller 470. Between the ITO transparent electrode and the liquid crystal 465, the plate 455 is further coated with an alignment layer. The alignment layer is physically treated, typically by rubbing or buffing, to induce a direction at which the molecules of the liquid crystal material tend to align. This alignment direction determines the director orientation of the liquid crystal and is used to orient the liquid crystal element 465 relative to the polarized light being transmitted.

Spacer 460 is provided to maintain the thickness of the zone between the alignment layers 455, occupied by the liquid crystal material 465. The spacer 460 is shown as a sphere but could be a different shape such as a cylinder or the like. The spacer can be a polymer or silica glass of the type produced in large numbers with relatively uniform size. Spacers 460 can be admixed into the liquid crystal material to provide a minimum thickness to which the liquid crystal 465 can be compressed. The spacer materials may be around the periphery of the liquid crystal material in the element (inside the glue edge) or distributed through the liquid crystal material. The spacers can be used to keep the cell gap uniform (although it optionally can be intentionally made slightly slanted to avoid interference fringes arising from coherent laser light). The ratio of volumes of the spacer material to the liquid crystal material is low enough to minimize the effect on light propagating through the liquid crystal material. When the liquid crystal material is contained between the alignment layers, the molecules in the liquid crystal layer near plate 450 line up parallel to plate 450 and to the direction of alignment of alignment layer 455. The liquid crystal cell can be configured as an electrically controlled birefringence (ECB). The liquid crystal may also be a nematic or smectic liquid crystal.

In contrast to the uniform liquid crystal cell of FIG. 4A, FIG. 4B schematically illustrates a wedged-shaped liquid crystal cell of an electronically controlled birefringence cell. In the embodiment of FIG. 4B, spacers 460 and 461 are of different sizes, such that the plates 450 are not parallel and the overall cell has a wedge-shape. That is, spacers supporting the plates on opposite sides of the liquid crystal element are or different thicknesses, and as a result, the retarder in FIG. 4B defines a wedge with thickness that varies with respect to the point at which light passes through the retarder element. Light passing through the retarder element will encounter a different thickness at a point closer to one spacer or the other, and across the distance between the spacers will encounter a thickness that varies linearly (because the plates in this case are flat). Thus, retardation is a function of the relative diameters of the spacers (d₁, d₂) as well as the effective refractive index (n_(eff)) of the cell.

FIG. 4C is a schematic illustration of an optical device and an optical stage according to one embodiment of the disclosure. In the embodiment of FIG. 4C, optical device 400 is shown has having three optical stages. At least one of the stages 405 is arranged to provide for different effective retarder thickness across the X-Y operative area in which light can pass through the stage. The stage 405 shown has a uniform retarder 410 coupled to wedged liquid crystal 415. The wedged liquid crystal 415 receives complementary female wedge 420 to form a geometric cube. The wedged liquid crystal is shown to have two spacers 417. Conventional liquid crystal material can be used for this application. Voltage source 435 can be coupled to the liquid crystal segment of stage 400 to fine-tune the birefringence of stage 405. Uniform retarder 410 can be made of quartz, lithium niobate (LiNbO₃) or a polymeric material having the desired birefringent optical characteristics. Female wedge 420 can be a homogeneous optical transparent glass or a polymeric material having similar optical properties.

The placement of the female wedge is optional. Although in the exemplary embodiment of FIG. 4C, only one stage of the filter 400 is shown as having a wedge-shaped liquid crystal segment, it should be noted that the disclosure is not limited thereto and more than one stage can be configured according to the principles of the disclosure. Moreover, the principles of the disclosure are not limited to having a three-stage filter and may include more (or less) stages than shown in FIG. 4C. Indeed, the principles illustrated in the exemplary embodiment of FIG. 4C can be used with pixelized and/or columnized LCTF. The number of liquid crystal cells can be a function of the application. For example, for a Lyot filter, each stage may generally contain one liquid crystal cell while for an Evans-type filter each stage may contain multiple liquid crystal cells.

In one embodiment, the disclosure relates to a multistage filter where each stage includes a wedge-shaped liquid crystal cell and, optionally, a complementary optical transparent wedge-shaped glass. Moreover, the liquid crystal cell can be coupled to a voltage source to enable further fine-tuning of the cell. The filter can be tuned by using the voltage source and a controller.

In the embodiments with wedge-shaped elements as shown in FIGS. 4 and 5 and with stepped thickness elements as in FIG. 6, across an X-Y image field, there is a difference in thickness of the operative retardation element(s) along the light propagation axis Z, for different points in the X-Y field. The difference in thickness can occur in the controllable birefringent liquid crystal element. The difference in thickness alternatively or additionally can occur in a retarder of fixed birefringence, used with another element that is controllable for tuning. In each of these situations, light traversing the element along a line parallel to the Z axis at a given point in the X-Y image field, is subjected to a different phase delay, due to the difference in thickness Z, from the phase delays of other points in the X-Y image field. As a wavelength bandpass filter, a result of the wedge-shaped element is that the center wavelength that is passed by the filter differs at different points across in the X-Y field.

In the embodiments of FIGS. 4-6, the wedge shape has a thickness that is either linearly or stepwise varied from a minimum thickness at one extreme of one of the X or Y axis to a maximum thickness at the other extreme. The minimum thickness can taper to some non-zero minimum thickness or can taper to a sharp edge. It is also possible that the thickness can vary in both X and Y, for example with the minimum and maximum thickness occurring at opposite corners rather than adjacent corners. That structure would be equivalent to rotating the wedge element by 45° from the orientation shown. Other variations in thickness are potentially applicable to vary the wavelength passband at different points in a field, such as conical shapes, pyramids, truncated cones or pyramids, etc.

The thickness variation of wedge shape according to this aspect should be distinguished from the technique of slanting a birefringent element used with a monochromatic (laser) light source as a means to prevent fringing of a monochromatic image due to interference effects. According to the present invention, and unlike the anti-fringing technique, a thickness variation is introduced into the tunable or fixed birefringence elements so as to cause the device to tune to different wavelengths at different X-Y locations on an image field at the same time. This is accomplished in the exemplary embodiments with continuously wedge shaped tunable or fixed retarders (the wedge shape potentially resulting in a slanted surface or interface), or a step-wise wedge shape (e.g., FIG. 6).

FIG. 5 is a schematic illustration of an optical stage according to another embodiment of the disclosure. According to the exemplary embodiment of FIG. 5, optical stage 500 comprises uniform liquid crystal 510 coupled to wedge retarder 515 and female wedge 520. Spacers 517 are shown along an axis of the liquid crystal 510 device to maintain a uniform thickness. As with the exemplary embodiment of FIG. 4, female wedge 520 is complementary to wedge-shaped retarder 510 and may be used optionally. In addition, a voltage source 535 is provided to supply a programmed voltage to the uniform liquid crystal segment 510.

In another embodiment, the disclosure relates to a tunable filter comprising several uniform stages. Each uniform stage may include a fixed wedge-shaped retarder, and optionally, a homogeneous optical transparent wedged glass. The wedge-shaped glass can be configured to complement the wedge-shaped retarder so that when combined, the two form a three dimensional rectangle. Each stage may further include a first polarizer affecting photons coming into the retarder and a second polarizer affecting the photons leaving the optional transparent glass. The tunable filter may include a stack of N stages. In another embodiment, a stage according to the principles disclosed herein may be used as a part of a stack configured as a Lyot filter, a Solc filter, an Evan filter or a hybrid filter. As is known to one of skill in the art, a hybrid filter may contain many stages (at least two stages). Some of the stages in a hybrid filter may include Lyot filter, Solc filter, Evans filter or Fabry-Perot interferometer.

FIG. 6 schematically illustrates a stepped LCTF device for un-patterned Raman imaging or other chemical imaging applications according to another embodiment of the disclosure. In the exemplary embodiment of FIG. 6, stage 600 includes uniform liquid crystal 610 optically communicating with retarder 615 having a stepped profile (herein “stepped retarder”). The stepped retarder 615 receives a complementary female wedge 620. As with the exemplary embodiments of FIGS. 4 and 5, stage 600 may be coupled to voltage source 635 for better optical modulation. The embodiment of FIG. 6 can be modified to include a symmetric uniform retarder, a stepped liquid crystal and a complementary female wedge. As before, the female wedge can be a homogeneous optical transparent glass or a composite having similar optical properties.

In one embodiment, a tunable filter can be configured to include N stages. Each stage can have a male stepped-retarder and a complementary female homogeneous transparent glass. A polarizer can cover each face of the three dimensional rectangle formed by combining the retarder and the complementary glass. A plurality of stages can be assembled in an order of increasing thickness to form a tunable filter. In this embodiment, the filter resolution is determined by the number of stairs as well as the number of stages in the filter.

FIG. 7 is a schematic illustration of a LCTF device for Raman imaging or other chemical imaging applications in which the thickness variations occur in striated bands that extend across the X-Y field of the filter element, horizontal band or striations being shown. FIG. 7 shows a three-stage Lyot-type filter according to one embodiment of the invention and is similar to the previous embodiments in that each stage may include a liquid crystal cell and an optional retarder element such as a birefringent crystal. Each stage is shown to be interposed between two polarizers (at entry and exit points of the stage). The birefringent crystal and liquid crystal have optical axes aligned at 45° to the orientation of an input polarizer (the initial polarizer not shown in FIG. 7). The conductive layer (normally indium tin oxide) can be provided on at least one glass substrate of the liquid crystal cell patterned in rows (or columns) of a given width. Moreover, each row or column can be coupled to the same or a different control voltage by means of individual control voltage sources V₁ to V_(n). In one embodiment, the columns (or rows) of each stage are aligned with similarly situated columns (or rows) of other stages. Referring to FIG. 7, stage 710 is receiving voltage sources V₁, V_(i) . . . V_(n). The filter can be used as a conventional tunable filter by setting the voltages to each column equal in the same stage, and the filter can be arranged to tune each column or row to a different voltage for providing a different wavelength bandpass. The successive stages 710, 720, 730 are operated in coordination to pass the same wavelength through each aligned column (or row) in the stacked stages.

By applying different voltages across various columns (or rows) of the liquid crystal cell of stage 710 and related columns (or rows) in the following stages 720, 730 such that a substantially independently tunable filter configuration is formed through the columns (e.g., the Lyot configuration with thicknesses R, 2R, 4R), a different bandpass (λ₁-λ_(n)) can be defined at different columns (which is shown in FIG. 8. and is further discussed below.) This translates the imaging LCTF into a dispersive spectrometer. Although the first stage 710 is shown as coupled to voltage sources V₁ to V_(n), the principles of the disclosure are not limited thereto and stage 710 can be devised to receive only one voltage source. Moreover, the voltage sources can be applied along the various columns of the liquid crystal segment of stage 710. Applying a voltage source is not limited to only one stage of the filter and can be devised such that each of the various stages are driven by a voltage source. The various columns (or rows) can also be binned together so that part of the image received from CCD is at, for example, wavelength 1 while another part of the image received from CCD is at wavelength 2. Stage 710 can also include a uniform retarder, a stepped or a wedge retarder and a complimentary wedge as described above. A black matrix mask can be placed on the exit polarizer of the last stage to prevent light leakage at inter-pixel regions or inter-column regions, when the filter is not tunable.

FIG. 8 is a schematic illustration of a columnized LCTF device for Raman or other chemical imaging applications. Referring to FIG. 8, first stage 810 is coupled to several voltage sources V₁-V_(n). Each of voltage sources V₁-_(n) may operate independently of the others to supply power to electro-optically tuned liquid crystal cell of stage 810. This results in a dynamic LCTF with high spatial resolution and a filter that can be operated to distinguish by image or by wavelength. By providing different voltages across the column (or rows) of the liquid crystal segment as well as the related columns (or rows) in the subsequent stages, the tunable filter can be reconfigured along the X axis to provide a 1-D spectrum (Y, λ) at certain location X as a function of the activation state of the LCTF. The liquid crystal used in the columnized system of FIG. 8 can be either nematic or smectic. Although the embodiment of FIG. 8 depicts only one of the three stages as receiving separate voltages, the disclosure is not limited thereto and stages 820 and 830 may also be configured for columnized tuning.

The filter of FIG. 8 can be coupled to a photon emission source, a photon detection source, an optical lens and a processor to form a system for obtaining a spatially accurate wavelength-resolved image of a sample having a first and a second dimension. The photon detector may include a charge-coupled device, a complementary metal oxide semiconductor, a charge injection device, an intensified charge injection device, an electron multiplying charge-coupled device, a silicon photo diode, a silicon avalanche diode and a focal plane array. The photon emission source may be laser, a light emitting device or a fluorescence device.

FIG. 9 is a schematic illustration of an un-patterned Raman imaging or other chemical imaging applications system according to another embodiment of the disclosure. Referring to FIG. 9, optical filter 900 receives scattered photons from object (optionally a Raman Object) 905 and produces spectrum 910. The wedged-shaped liquid crystal filter 900 may be coupled to a voltage source 907. Applying voltage 907 to filter 900 changes crystal retardation at positions along filter 900. By applying a voltage to filter 900 at various positions along the lateral length of the filter, filter 900 can produce variable retardation. In an optional embodiment, each of rows 1 through N can receive a different voltage.

In FIG. 9, the wedge design can create a series of bandpass zones. Each bandpass zone may allow a different wavelength (λ₁-λ_(n)) through at different locations of filter 900 in FIG. 9. Similar to the exemplary embodiments of FIGS. 4 and 5, the bandpass zones may be disposed in horizontally in the X direction. The different bandpass zones allow filter 900 to operate as a dispersive spectrometer. The resolution of the filter can be a function of the CCD camera's pixel size and the wedge angle of the filter for each stage (if a multi-stage filter is utilized). The additional liquid crystal cell in each stage can add the feature of tunability to the filter so that each bandpass zone of the filter can be tuned independent of the other zones. As compared with the embodiments of FIGS. 4C and 5, where voltage is applied to the entire liquid crystal cell such that retardation of each stage satisfies a Lyot filter configuration (i.e., R, 2r, 4R, etc.), the embodiment of FIG. 9 can be configured to generate a series of bandpass zones (λ₁-λ_(n)) at different positions of the filter along the X direction (for example at locations X₁-X_(n) not shown). The retardation of the liquid crystal cell may be changed by changing the applied voltage thereby allowing the liquid crystal cell to act as a variable retarder. Therefore, at each position X_(i) (not shown) the bandpass wavelength can be changed by changing the voltage to the liquid crystal cell. The final two-dimensional image 910 at a particular wavelength λ_(i) may be constructed by a computer by combining, for example, the bandpass zones that are passing the same wavelength λ_(i).

FIG. 10 is a schematic illustration of a compact imaging filter for the handheld system using a tunable Fabry-Perot micro opto-electromechanical system. The sample 1005 shown here as the numeral 2 is an opaque surface which is irradiated by light and either reflectively scatters incident light or absorbs light energy and re-emits the energy at characteristic wavelengths, in all directions. The rays emanating from an arbitrary point (x,y) on sample 1005. An optical system generally represented in the drawing by lens 1009 directs rays from the point (x,y) in a direction 1012 through one or more compact 2-D Raman imaging filter elements 1010, two being shown in the example. A receiving optical system generally represented in the drawing by lens 1011 is used to obtain a spatially accurate wavelength resolved image of the sample, on the surface of a detector 1020.

In FIG. 10, the imaging filter comprises a series of filter elements having paired thin partially transparent parallel Si plates separated by air so as to cause the pair(s) each to define an optical cavity. The plates can be fabricated by micro-machining and are movable by a micro-electromechanical positioners (not shown) to adjust the cavity thickness between the plates and thereby to tune the resonant wavelength of the cavity. Preferably, two or more pairs of substantially reflective (but less than 100% reflective) plates 1010 form the cavities of one or more Fabry-Perot interferometers. The plural cavities are set to the same cavity spacing. Rays oriented normal to the plates at the resonant wavelength are passed and other wavelengths are reflected backwards along the optical path.

Optical plates 1010 can be fabricated from a planar Si wafer and the wafer can be configured to form a supporting frame by micro-machining steps, for example using chemical or ion beam etching. By additional processing steps known to those skilled in the art, connecting members are formed between the supporting frame and the thin substantially reflective Si plate (e.g., along the sides and corners of each Si filter element) so as to provide electronically controllable positioning actuators for setting the cavity thickness(es). These actuators (not shown) move one or both optical plates 1010 in pairs defining the Fabry-Perot interferometers. The plates 1010 can be moved uniformly in and out of the plane of the respective supporting frame (i.e., in a direction parallel to the optical axis and normal to the parallel planes of the optical plates), for setting the cavity thicknesses as indicated by the arrows.

In one embodiment, the actuator can be activated by a voltage source. The actuator can be switchable between discrete tuning positions (cavity thicknesses) or capable of adjustment to a desired point in a range of actuator displacement positions, depending on the desired outcome. This set of planar structures containing the Si filter elements and onboard actuators are stacked along the optical axis to form a Fabry-Perot imaging filter. In this stack each thin parallel Si plate is parallel to and aligned with all the other plates so as to all point in a direction parallel to a central optical axis.

The final number of plates can define Fabry-Perot etalons between pairs, or can be odd if the plates of successive pairs are also used as a resonant cavity and thus define another pair. In that event, the spacing of the plates and the spacing between such successive pairs must each be controlled to employ the same resonant spacing. Different combinations of MEMS actuator displacements between pairs of filter elements together with combinations of different stacking distances between sets of plates (such as to form stages) will allow the transmission of a narrow passband of light at a selected wavelength in a range of wavelengths to which the device is controllably tunable.

The filter wavelength λ is selected by changing the cavity or distance between each of the Fabry-Perot filter elements in a predetermined manner. The actuator displacements, the spacing between plates and number of plates determines the range of the wavelengths over which transmission is achieved upon actuator changes (i.e., tuning), as well as the wavelength window over which the light at wavelength λ is transmitted (i.e., the bandpass.) Bandpasses for such a device can be as narrow as 0.25 nm (high resolution) or up to 10 nm (lower resolution). The ranges of wavelength over which the device operates (i.e., filter different wavelengths of light) can be, for example, between 400 nm and 1800 nm. The design tradeoffs that achieve such performance are selected to optimize the number of elements (cost and simplicity) and the overall transmission function (optical efficiency) for any particular measurement requirement such as Raman, Fluorescence, VIS or NIR chemical imaging.

For Raman, Fluorescence, Visible or NIR operation a particular arrangement of stacks of the plates can be used for which a predetermined set of actuator voltages are known that provide the required wavelength filtering characteristic. For the particular mode of operation the required set of voltages are then called by the computer and applied to the actuators for each wavelength to be imaged. Scanning a set of actuators and acquiring the data over the full image 1020 creates the wavelength resolved spatially accurate image.

Preferably, a 2-D image (X, Y) image of the sample is produced on the detector 1020. The tunable optical cavities produce the image at one wavelength at a time and are controllable for tuning to two or more wavelengths and optionally to selected wavelengths in a range. The tuning speed can be less than 1 sec for changing between cavity spacings and thus selecting for an image at a new wavelength.

The optical systems 1009 and 1011 before and after the Fabry-Perot plates can be used to allow the light scattered or emitted from the sample 1005 to accurately and faithfully reproduce each (x, y) location of the sample 1005 onto the detector plate 1020 at (x′, y′). The specific detector pixels form an image or spatially accurate representation corresponding to positions of points in the image of the sample. Optical configurations are possible where transmitted light is discriminated without using optical systems 1009 and 1011 that contain lenses. For example, a stacked Fabry-Perot array wherein each etalon discriminates for wavelength due to the thickness of the cavity in a normal direction has the characteristic of selectively passing only normally oriented rays, thereby effectively collimating the light from the image and providing a spatially resolved image as applied to the detector.

The embodiment of FIG. 10 enables an ultra-compact high resolution Raman or fluorescence imaging device which can be selectively fine-tuned to enable acquisition of a Raman or fluorescence spectrum corresponding to each spatial element of the sample. Individually addressable Fabry-Perot filter elements 1010 can enable acquisition of a Raman or luminescence spectrum corresponding to the spatial elements of sample 1005. When detector 1020 is a CMOS detector, each individual pixel can be sensed independently by tuning the wavelength applied to specific pixel elements which is not possible in typical CCD detector device. Use of such a Fabry-Perot imaging filter is novel in that prior MOEMS based Fabry-Perot Filters have not been capable of performing imaging—only wavelength selection of an optical source. An additional advantage in this invention is that the CMOS detector in this preferred embodiment can be pixel selected so as to apply attention only on the important pixels in the sample—thereby speeding and simplifying collection of the most important data from specific regions of the sample.

The Fabry-Perot filter element not only transmits but in an alternate embodiment can reflect the light to an individual pixel of a CMOS sensor to form a single wavelength Raman or luminescence imaging object 1020 of spatially accurate spectrally resolved pixels of the sample. That is, in an alternative embodiment one or more Fabry-Perot filter elements can be used as a reflective wavelength filter rather than a transmissive one.

The wedged or stepped-shaped birefringence interference filters, the MOEMS device and the dispersive spectrometer disclosed herein can be made very compact and are particularly suitable for use in a handheld imaging system. Moreover, the filters can be configured to operate in two modes: imaging mode and spectroscopic mode. The tuning method can include line scan in a ID spectrum. Thus, at a certain location X, the sample can be scanned as a function of Y and λ. The tuning speed can be as low as about 20 ms per wavelength or less than about 1 sec. per scanning line.

In this respect, a “compact” or “handheld” or “portable” version should be considered to comprise a self powered unit of the approximate size of a handheld calculator, cell phone, PDA or the like, namely of a size that can be carried conveniently in a pocket and deployed wholly by hand. Preferably the handheld device is about 36 in³ or less (3×6×2 in) and can be as small as 9 in³ (3×6×0.5 in) or less and the optical path measured from detector to the sample is about 2-4 inches.

In a handheld system using the tunable filter configurations disclosed herein may include a controller for controlling the tunable filter and the CCD. The controller may be in the form of a processor programmed by a software to communicate with the operator through a keypad and a display unit. The handheld system may also include a photon emission source, a polarization beam splitter, and a power source. The power source can be a battery. The photon emission source can include a laser (for Raman scattering), an LED (for white light reflectance application or fluorescence emissions), near infrared source, a fluorescent source or a combination thereof. The handheld device may also include one or more rejection filters for preventing the emission source from interfering with the LCTF and the detector.

FIG. 11A is a schematic illustration of a handheld Chemical Imaging Threat Assessor device according to one embodiment of the invention. Referring to the exemplary embodiment of FIG. 11A, the CHITA device includes miniaturized components efficiently packaged into a portable compact form for hand held operation. The unit includes illumination/excitations sources (laser source 1110 and light emitting diode source 1105), a lens and reflecting surface 1114, a scattered/emitted/reflected light conditioning filter 1107, a polarization beam splitter 120, a combination of filter 1124 (e.g., a fluorescence liquid crystal tunable filter (FLC)) and detection system 1126 (e.g., CMOS or CCD) for wideband detection of sample region screening and selection, a second combination of filter 1130 (e.g., a Raman liquid crystal tunable filter (RLC1 and RLC2)) and detection subsystem 1132 (e.g., CMOS or CCD) for narrowband detection and identification, the control electronics 1134, processor unit 1136 and battery 1140. Imaging filter 1124 (FLC) is a fluorescence LCTF. Imaging filters 1130 (RLC1, RLC2) may be Raman LCTF. Conditioning filter 1107, which may be a “notch filter”, is positioned before polarization beam splitter 1120 to block laser light from overwhelming the beam splitter 1120 and the subsequent detection systems 1124, 1130. Additionally, lenses 1122 and 1128 are positioned between beamspilitter 1120 and liquid crystals 1124 and 1130, respectively.

In the exemplary device of FIG. 1A, laser 1110 provides photonic beam 1112 which can be used for narrowband excitation and analysis including Raman analysis. The first filter/detection subsystem including emitting diodes (LEDs) can be used for screening purposes.

FIG. 12 is a secondary illumination source according to one embodiment of the disclosure. In FIG. 12, the LED's are configured in a concentric ring 1200 normal to an axis that intersects the sample. In other words, they are formed as a ring 1200 and positioned in the vicinity of the sample (e.g., 1105 at FIG. 11A) to illuminate the sample with photons. The embodiment of FIG. 12 shows diode illuminating ring 1200 having LED's 1205 and 1210 which may illuminate the sample with different wavelengths. In other words, ring 1200 may include an array of similar of different light emitting diodes for irradiating the sample to enable sample detection and identification. In addition, the sample illumination may enhance laser detection and identification. LEDs 1205 and 1210 can be selected to operate in the visible, NIR or UV bands depending on the desired application. They may also be selected to consist of a mixture of different diodes selected for screening a combination of different chemical warfare or biological agents. The illumination source of FIG. 12 may also be configured to be a fluorescence source.

Referring again to FIG. 11A, the sample may be illuminated and analyzed substantially simultaneously or sequentially with laser from source 1110 and emitted photons from LED source 1105. Next, the scattered and emitted photons beam 1118 from the sample are collected by a lens 1116 and reflected by surface 1114 into the wavelength imaging filter. The reflecting surface 1114 can be configured to allow the laser light to pass thru while reflecting the scattered and emitted photon beam 1118. The illumination filter 1107 removes the wavelengths of the Raleigh scattered illuminating light that can swamp the detectors. The polarization beam splitter 1120 separates one polarization of the emitted and scattered light and allows it to be analyzed by the two different filters/detector subsystems shown to the left and below beam splitter 1120. Either CMOS (shown) or CCD detectors (1132, 1126) may register the wavelength selected/filters signals that are read and analyzed by the control module 1134 and an analyzer. The analyzer can be software stored on the CPU using data stored in the memory of the device. Alternatively, a processor can be programmed with a software to detect the chemical signature of the sample by comparing its spectrum with known spectrums stored in the CPU database.

FIG. 11B shows a different side of the handheld device of FIG. 11A. Referring to FIG. 11B, device 1100 includes battery source 1140, keypad or other interface device 1152 and screen 1150. Screen 1150 may be configured for displaying the resulting images, or spectra and a bio-threat warning indication when such a threat is identified. The visual indicator can be complemented by an audio warning signal or other identification means. Keypad 1152 may be used for control and inputting data or for addressing commands to unit 1100. The device may also include one or more communication ports for electronically communicating with other electronic equipments such as a server, a printer or the like.

Device 1100 may be configured to illuminate or irradiate the sample and collect and analyze photons emitted or scatted by the sample. Analysis and identification can be accomplished as a function of the wavelengths emitted or scattered by the sample (i.e., the spectra of the sample). Thus, a spectra similar to that produced by a spectrometer can be obtained. In another embodiment, The principles disclosed herein are particularly advantageous in that device 1100 enables obtaining spatially resolved images as well as spectral identification of a sample simultaneously. In another embodiment, the disclosure relates to a portable system for obtaining a spatially accurate wavelength-resolved image of a sample having a first and a second spatial dimension. The portable system can include a photon emission source for sequentially illuminating a plurality of portions of said sample with a plurality of photons to produce photons scattered by the sample. The photon emission source can illuminate the sample along the first spatial dimension for each of plural predetermined positions of the second spatial dimension. The system may also include an optical lens for collecting the scattered photons to produce therefrom filtered photons, a dispersive spectrometer for determining a wavelength of ones of the filtered photons, a photon detector for receiving the filtered photons and obtaining therefrom plural spectra of said sample, and a processor for producing a two dimensional image of said sample from the plural spectra.

FIG. 11C is a schematic illustration of a handheld CHITA device according to another embodiment of the invention. Specifically, FIG. 11C shows an embodiment where filters 1124 and 1130 are each a dispersive spectrometer. In another embodiment, at least one of the filters 1124 or 1130 is replaced by a dispersive spectrometer.

FIG. 13 schematically illustrates the operation of an exemplary CHITA device. As shown, a sample is first illuminated 1305 by one or more photon sources (e.g., LED 1105 and laser 1110 in FIG. 11A). The next step, 1310, is the collection and analysis of the emitted and/or scattered photons from the sample. Once the wavelength and spatially resolved data/information is obtained, in step 1315 the data is directed to a CMOS or CCD detector. This data is then processed by the CPU to perform various corrections to the raw data for detailed analysis. The correction analysis may include calibration and baseline corrections 1320, unmixing of the different spectral features 1325 (such as those disclosed in U.S. patent application Ser. No. 10/812,233 and incorporated herein for background information), searching a local database for potential spectral matches 1330 and identification of the sample 1335. The analysis may be repeated for different spatial locations on a sample. The spectral data can be stored in the CPU 1136 or compared with baseline data stored in an onboard memory. The exemplary processing steps enable detected and identification of compounds classified as bio-threat or chemical threat 1340.

According to the foregoing embodiments, pixilized or otherwise incremental parts of the tunable filter are independently tunable, and can be tuned wholly independently or in a coordinated way to filter selectively for the wavelengths to be applied to the data capture devices at different points in the image or data capture field (e.g., an X-Y field of photosensors or a line of photosensors in a pushbroom configuration). In the stepped-thickness retarder embodiments, for example, a succession of pixel areas or bands are tunable together, so as to provide a succession of incremental areas (at each step) tuned to successive wavelengths that differ according to the difference in retarder thickness from one step to the next. Similarly, in a wedge shaped retarder embodiment, tuning over the wedge selects for a succession of wavelengths in a range, the wavelength varying continuously across the surface at which the retarder thickness varies between its greatest and least thickness. In a wholly pixilized filter, the individual pixels could be tuned to wavelengths that are different from the tuned wavelengths of other potentially-adjacent pixels.

It is an aspect of the invention, therefore, that different positions on the tunable filter area are tuned to different wavelengths at the same time. This is a departure from the expected technique of collecting wavelength specific light amplitude data over the entire filter area at one wavelength, and then proceeding to collect a next set of data at a next wavelength until the entire spectrum is collected for each pixel position. However, the invention provides improved speed and versatility by foregoing the need to collect the full spectrum (all the wavelengths) for the full tunable area before an analysis of the data can be accomplished.

It is possible using the independent or stepwise tuning capability of the invention to collect full spectrum information at each pixel position. In that case, it is necessary to manage the data collection so as to keep track of the tuned wavelength for which each light amplitude measurement applies.

According to the invention, it is also possible to collect a series of different wavelength measurements from a sample at one time. This can be accomplished, for example, by de-focusing or otherwise applying the reflected light of an image diffusely over all the tunable positions in an array (e.g., a wedge or series of steps of different retarder thickness) and accumulating wavelength data from the whole sample in a manner similar to the manner in which light from a slit might be applied to a spectrograph using a prism or grating. That is, the wedge or stepped or pixilized retarder arrangements can be operated as to collect an average spectrum for a whole sample image. The same arrangements also can be used to collect an average spectrum over a selected part of an image.

In one example, the tunable filter is controlled by a processor such as processor 1136 in FIG. 11A, and the processor also controls successive data acquisition modes. As one step, focused fluorescence imaging is used to collect an image of the sample. One or more particular areas of the image can be distinguished by detection of features of interest, e.g., by contrast at some characteristic wavelength. In a next mode, the area of interest, or optionally diffuse light from the entire sample, is examined for particular wavelength relationships, optionally using very narrow bandwidth (but slow) Raman imaging. Inasmuch as it is not necessary to collect a full spectrum of data for every pixel or other tunable increment, the sample analysis can be substantially faster than would be possible otherwise.

The dispersed wavelength capabilities of the wedge shaped, step shaped, and separately tunable retarders of the foregoing embodiments thus can accomplish spectral analysis by collecting full spectra for pixels, or spectral analysis by collecting an average spectrum of the image, or differential wavelength analysis wherein adjacent or other areas of the image are selective tuned to different wavelengths.

Accordingly, in one embodiment, a handheld detection system, for threat detection or other applications, can be configured and controllably operated to use a birefringent spectrally “agile” interference filter element, namely an element that is tunable selectively to one or to a plurality of different wavelengths at a given time, to accomplish fluorescence imaging, reflectance image collection, Raman image and Raman image-average spectrum collection and analysis. Any one or a combination of the filter layouts disclosed herein can be used in the handheld device. For example, referring to FIG. 11A, RLC1 and RLC2 (Raman liquid crystal) may be a wedged filter or a tunable filter as described above. Similarly, FLC 1124 may include a dispersive filter as disclosed above.

FIG. 14 provides exemplary packaging options according to one embodiment of the disclosure. Referring to FIG. 14, device 1410 illustrates a handheld device suitable similar to that shown in FIGS. 11A and 11B. The handheld device 1410 includes screen 1412 for displaying various information with the operator, alarm signals 1411 and communication port 1413 enabling data communication with other electronic devices. Device 1400 can be used for bio-threat detection as it can display images as well as text. This unit can be configured to identify a large range of bio-threat material determined by the size of the onboard library of bio-threat signatures contained in its memory. It may also be configured for remote communication with a host station using a wireless link to report important findings or update its library.

Handheld air monitor 1420 is shown to include port 1422. Finally, handheld surface detection device 1430 is shown to have handle 1433, LED source 1431 having a ring formation and body 1432. While not shown, handheld surface detection device may also include a display, a keypad and one or more communication ports. The devices shown in FIGS. 11A, 11B and 14 illustrate that the embodiments disclosed herein can be assembled and packaged in a handheld device for field application. It can be readily seen that such devices are compact and while having a small optical path (measured between the sample and the detector) can be as efficient as the table-top units. Devices 1420 and 1430 are particularly suitable for air monitoring or surface detection of bio-threats, respectively. Handheld detector 1430 can be used for measuring bio-threats on the clothing or exposed body parts, which can be particularly useful on the battlefield or in a civil setting

While the exemplary embodiment of FIG. 14 are discussed in relation with bio-threat detection, such device are equally suited for chemical warfare agent detection or hazardous material monitoring. Another application of detection device 1410 may include detection and monitoring of chemicals in human body for medical purposes. A consumer device using the principles disclosed herein can also be configured for conducting self-diagnostic tests for identifying agents such as glucose, cholesterol, urea, hemoglobin and alcohol.

Finally, handheld device 1440 represents an exemplary representation of a lower cost, consumer oriented device with simplified operating controls and menu driven input similar to a cellular telephone. This unit can be programmed to detect a certain chemicals depending upon its intended application.

An alternate embodiment for the handheld detector involves the use of a Raman micro-spectrometer as the dispersing filter for Raman scattering. To achieve the small size required for a portable handheld detector, the micro Raman sensor component can be constructed from semiconductor lithographic materials such as PMMA and x-ray lithography processes known to those skilled in the art of semiconductor processing. Fabrication involves exposure of polymethyl methacrylate (PMMA) fitted with an X-ray mask to synchrotron radiation. Here the exposures can be performed using an in-plane micro-optical systems known to those skilled in the art and fabricated in batch mode via deep x-ray lithography. Subsequent steps may involve development of the exposed PMMA and removal of same, electroformation in the PMMA cavity, planarization of the combined materials, removal of the protected PMMA, and finally release of the electroformed component. This process produces a PMMA grating which can be used as the miniature wavelength dispersive element for the CHITA handheld detector. Following the art for construction of dispersive spectrometers, this grating is inserted into the optical path so as to spread the filtered wavelengths over the detector surface to detect the range of wavelengths coming from the sample.

A feature of the ultra compact filter is that the optical properties and Raman features inherent to PMMA do not substantially degrade the Raman detection of bio-threat agents. FIG. 15 shows the Raman spectrum of PMMA collected using high performance microscope glass optics. FIG. 15 also shows the Raman spectrum of a species of Anthrax (BG) collected with the same optics. Finally, FIG. 15 shows a Raman spectrum of BG after introducing a thin plate of PMMA into the laser illumination and collection optical paths. So long as the PMMA is located in an optical region where the light rays are parallel, it does not introduce significant background into the measured spectrum. That is, PMMA is illumination optical path does not prevent collection of BG Raman spectrum. The contribution to the Raman spectrum attributable to PMMA can be also be used as an internal calibrant (intensity and wavelength) which can assist automated correction of instrument response and overall improved performance of a fielded system. Instrument calibration enables compensation of instrument variation including laser line drift and quantitative analysis.

The following Table shows non-exclusive and exemplary specifications for an embodiment of the compact imaging spectrometer. Performance Parameter Specification Laser Excitation wavelength 532 nm; <0.2 nm and bandwidth Imaging aperture: 0.5″ or larger Field of view (angular +/− 3 degrees incidence): Free spectral range: 500-750 nm Usable Raman range: 350-3,200 cm⁻¹ Resolution: 0.25 nm FWHM @ 500 nm, <10 cm⁻¹ Off-peak rejection: 10,000:1 total energy Trasnsmission min 30%

Although the disclosure is described using illustrative embodiments provided herein, it should be understood that the principles of the disclosure are not limited thereto and may include modification thereto and permutations thereof. 

1. In an optical filter comprising plural filter stages where a first one of the plural stages includes a retarder element and a liquid crystal cell each of which have a uniform thickness, the improvement of replacing the first one stage with a second one stage having a retarder element and a liquid crystal cell where either the element or the cell does not have a uniform thickness.
 2. The filter of claim 1 wherein the element and the cell of the first one stage have the same thickness.
 3. The filter of claim 1 wherein the element of the second one stage does not have a uniform thickness.
 4. The filter of claim 1 wherein the element of the second one stage is wedge-shaped.
 5. The filter of claim 1 wherein the element of the second one stage has a stepped-wedge shape.
 6. In an optical filter comprising plural filter stages where a first one of the plural stages includes a retarder element and a liquid crystal cell each of which have a uniform thickness, the improvement of replacing the first one stage with a second one stage having a retarder element and a liquid crystal cell where the cell comprises: a pixelized liquid crystal structure for filtering photons; a voltage source operably connected to said pixelized liquid crystal structure; and means for controlling the amount of voltage applied by said voltage source to ones of the pixels; wherein the liquid crystal is substantially uniformly thick in the direction of photon travel therethrough.
 7. In an optical filter comprising plural filter stages where a first one of the plural stages includes a retarder element and a liquid crystal cell each of which have a uniform thickness, the improvement of replacing the first one stage with a second one stage having a retarder element and a liquid crystal cell where the cell comprises: a columnized liquid crystal structure for filtering photons; a voltage source operably connected to said columnized liquid crystal structure; and means for controlling the amount of voltage applied by said voltage source to ones of said columns; wherein the liquid crystal is substantially uniformly thick in the direction of photon travel therethrough.
 8. An optical filter comprising: a first filter stage comprising a first retarder element and a first liquid crystal cell; and a second filter stage, wherein either the first element or the first cell do not have a uniform thickness.
 9. The filter of claim 8 wherein said second stage comprises a second retarder element having a first uniform thickness and a second liquid crystal cell having a second uniform thickness.
 10. The filter of claim 9 wherein the first uniform thickness and the second uniform thickness are substantially the same. 11 The filter of claim 8 wherein the first element does not have a uniform thickness.
 12. The filter of claim 11 wherein the first element is wedge-shaped.
 13. The filter of claim 11 wherein the first element has a stepped-wedge shape.
 14. The filter of claim 8 wherein the first and second stages comprise a birefringence interference filter.
 15. The filter of claim 8 wherein the first and second stages comprise an Evans split element filter.
 16. The filter of claim 8 wherein the first and second stages comprise a Solc filter.
 17. The filter of claim 8 wherein the first and second stages comprise a Lyot filter.
 18. The filter of claim 17 including an Evans split element filter.
 19. The filter of claim 8 wherein said first element is comprised of quartz.
 20. The filter of claim 8 wherein said first element is comprised of lithium niobate.
 21. The filter of claim 8 wherein the filter is a tunable filter.
 22. The filter of claim 8 wherein the filter includes a polarizer.
 23. The filter of claim 8 wherein the filter is pixelized.
 24. The filter of claim 8 wherein the filter is columnized.
 25. The filter of claim 8, wherein the first and the second filter stages define a hybrid filter.
 26. The filter of claim 8, wherein at least one of the first and the second stage is selected from a group consisting of a Lyot filter, an Evans filter, a Fabry-Perot filter and a Solc filter and the combination of the first and the second stage defines a hybrid filter.
 27. A method of manufacturing an optical filter comprising the steps of: providing a first filter stage comprising a first retarder element and a first liquid crystal cell wherein either the first element or the first cell do not have a uniform thickness; providing a second filter stage; and aligning the first filter stage with the second filter stage.
 28. The method of claim 27 wherein the first and second stages comprise a Lyot filter and including the steps of: providing an Evans split element filter; and aligning the Evans split element filter with the Lyot filter.
 29. The method of claim 27 wherein the first and second stages comprise a Lyot filter and including the steps of: providing a Fabry-Perot filter; and aligning the Fabry-Perot filter with the Lyot filter.
 30. The method of claim 73 wherein the first and second stages comprise a Lyot filter and including the steps of: providing a Solc filter; and aligning the Solc filter with the Lyot filter.
 31. The method of claim 27, wherein the optical filter is a hybrid filter and at least one of the first or the second stage is selected from the group consisting of a Lyot filter, an Evans Filter, a Solc filter and a Fabry-Perot filter.
 32. An optical filter for passing photons therethrough, the filter comprising: a first filter stage comprising a first retarder element and a first liquid crystal cell; and a second filter stage, wherein said first element comprises an input face and an output face, and wherein one of said first element faces is not oriented substantially normal to the trajectory of photons passing through the filter.
 33. The filter of claim 32 wherein said second stage comprises a second retarder element having a first uniform thickness and a second liquid crystal cell having a second uniform thickness.
 34. The filter of claim 33 wherein the first uniform thickness and the second uniform thickness are substantially the same.
 35. The filter of claim 32 wherein the first element is wedge-shaped.
 36. The filter of claim 32 wherein the first element has a stepped-wedge shape.
 37. The filter of claim 32 wherein the first and second stages comprise a birefringence interference filter.
 38. The filter of claim 32 wherein the first and second stages comprise an Evans split element filter.
 39. The filter of claim 32 wherein the first and second stages comprise a Solc filter.
 40. The filter of claim 32 wherein the first and second stages comprise a Lyot filter.
 41. The filter of claim 40 including an Evans split element filter.
 42. The filter of claim 32 wherein said first element is comprised of quartz.
 43. The filter of claim 32 wherein said first element is comprised of lithium niobate.
 44. The filter of claim 32 wherein the filter is a tunable filter.
 45. The filter of claim 32 wherein the filter includes a polarizer.
 46. The filter of claim 32 wherein the filter is pixelized.
 47. The filter of claim 32 wherein the filter is columnized.
 48. An optical filter comprising: a first filter stage comprising a first retarder element and a first liquid crystal cell; and a second filter stage, wherein said first element comprises an input face and an output face, and wherein said first element faces are not oriented substantially parallel to each other.
 49. The filter of claim 48 wherein said second stage comprises a second retarder element having a first uniform thickness and a second liquid crystal cell having a second uniform thickness.
 50. The filter of claim 49 wherein the first uniform thickness and the second uniform thickness are substantially the same.
 51. The filter of claim 48 wherein the first element is wedge-shaped.
 52. The filter of claim 48 wherein the first element has a stepped-wedge shape.
 53. The filter of claim 48 wherein the first and second stages comprise a birefringence interference filter.
 54. The filter of claim 48 wherein the first and second stages comprise an Evans split element filter.
 55. The filter of claim 48 wherein the first and second stages comprise a Solc filter.
 56. The filter of claim 48 wherein the first and second stages comprise a Lyot filter.
 57. The filter of claim 48 including an Evans split element filter.
 58. The filter of claim 48 wherein said first element is comprised of quartz.
 59. The filter of claim 48 wherein said first element is comprised of lithium niobate.
 60. The filter of claim 48 wherein the filter is a tunable filter.
 61. The filter of claim 48 wherein the filter includes a polarizer.
 62. The filter of claim 48 wherein the filter is pixelized.
 63. The filter of claim 48 wherein the filter is columnized.
 64. The filter of claim 48, wherein the optical filter is a hybrid filter and at least one of the first or the second stage is selected from the group consisting of a Lyot filter, an Evans Filter, a Solc filter and a Fabry-Perot filter.
 65. A system for obtaining a spectrum of a sample, comprising: a photon emission source for illuminating the sample with a plurality of photons to thereby produce photons scattered by the sample; an optical lens for collecting the scattered photons; a filter for receiving the collected scattered photons and providing therefrom filtered photons; and a photon detector for receiving the filtered photons and obtaining therefrom a spectrum of the sample, wherein said filter comprises: a first filter stage comprising a first retarder element and a first liquid crystal cell; and a second filter stage, wherein either the first element or the first cell do not have a uniform thickness.
 66. The system of claim 65 wherein said second stage comprises a second retarder element having a first uniform thickness and a second liquid crystal cell having a second uniform thickness.
 67. The system of claim 66 wherein the first uniform thickness and the second uniform thickness are substantially the same.
 68. The system of claim 65 wherein the first element does not have a uniform thickness.
 69. The system of claim 68 wherein the first element is wedge-shaped.
 70. The system of claim 68 wherein the first element has a stepped-wedge shape.
 71. The system of claim 65 wherein the first and second stages comprise a birefringence interference filter.
 72. The system of claim 65 wherein the first and second stages comprise an Evans split element filter.
 73. The system of claim 65 wherein the first and second stages comprise a Solc filter.
 74. The system of claim 65 wherein the first and second stages comprise a Lyot filter.
 75. The system of claim 74 including an Evans split element filter.
 76. The system of claim 65 wherein said first element is comprised of quartz.
 77. The system of claim 65 wherein said first element is comprised of lithium niobate.
 78. The system of claim 65 wherein the filter is a tunable filter.
 79. The system of claim 65 wherein the filter includes a polarizer.
 80. The system of claim 65 wherein the filter is pixelized.
 81. The system of claim 65 wherein the filter is columnized.
 82. The system of claim 65 wherein said photon detector is selected from the group consisting of: charge-coupled device, complementary metal oxide semiconductor, charge injection device, intensified charge injection device, electron multiplying charge-coupled device, silicon photo diode, silicon avalanche diode, and focal plane array.
 83. The system of claim 65 wherein said photon emission source is a laser.
 84. The system of claim 65 wherein said photon emission source is a light emitting diode.
 85. The system of claim 84 wherein said light emitting diode is disposed as a ring.
 86. The system of claim 85 wherein said light emitting diode is a plurality of light emitting diodes.
 87. The system of claim 84 wherein said light emitting diode is a plurality of light emitting diodes wherein one of said plural light emitting diodes emits photons at a wavelength that is different than a wavelength of photons emitted by another of said plural light emitting diodes.
 88. The system of claim 87 wherein the wavelength of photons emitted by one of said plural light emitting diodes is in the ultraviolet wavelength range.
 89. The system of claim 87 wherein the wavelength of photons emitted by one of said plural light emitting diodes is in the near infrared wavelength range.
 90. The system of claim 65 wherein said spectrum is obtained over a predetermined period of time and said sample is stationary during said predetermined period of time.
 91. The system of claim 65 wherein said scattered photons include photons emitted by said sample.
 92. The system of claim 65 wherein said spectrum is a Raman spectrum.
 93. The system of claim 65 wherein the system is a portable system.
 94. The system of claim 65 wherein the system is a hand-held system.
 95. The system of claim 65 wherein the spectrum is a Raman spectrum.
 96. The system of claim 65 wherein said spectrum is a spatially accurate wavelength-resolved image.
 97. The system of claim 65 wherein said spectrum is obtained over a predetermined period of time and said sample is stationary during said predetermined period of time.
 98. The system of claim 65 wherein said scattered photons include photons emitted by said sample.
 99. The system of claim 65 wherein ones of said filtered photons each have a wavelength within a predetermined wavelength band.
 100. The system of claim 65 wherein said first filter element includes a liquid crystal.
 101. The system of claim 65 wherein said retarder element is comprised of quartz.
 102. The system of claim 65 wherein said retarder element is comprised of lithium niobate.
 103. The system of claim 65, wherein the filter is a hybrid filter and at least one of the first or the second filter stage is selected from the group consisting of a Lyot filter, an Evans Filter, a Solc filter and a Fabry-Perot filter.
 104. A method for obtaining a spectrum of a sample, comprising: providing a sample; illuminating the sample with a plurality of photons to thereby produce photons scattered by the sample; collecting the scattered photons; receiving the collected scattered photons with a filter and providing therefrom filtered photons; and receiving the filtered photons and obtaining therefrom a spectrum of the sample, wherein said filter comprises: a first filter stage comprising a first retarder element and a first liquid crystal cell; and a second filter stage, wherein either the first element or the first cell do not have a uniform thickness.
 105. The method of claim 104 wherein the first and second stages comprise a Lyot filter and including the steps of: providing an Evans split element filter; and aligning the Evans split element filter with the Lyot filter.
 106. The method of claim 104 wherein the first and second stages comprise a Lyot filter and including the steps of: providing a Fabry-Perot filter; and aligning the Fabry-Perot filter with the Lyot filter.
 107. The method of claim 104 wherein the first and second stages comprise a Lyot filter and including the steps of: providing a Solc filter; and aligning the Solc filter with the Lyot filter.
 108. The method of claim 104, wherein the optical filter is a hybrid filter and at least one of the first or the second stage is selected from the group consisting of a Lyot filter, an Evans Filter, a Solc filter and a Fabry-Perot filter.
 109. An optical filter comprising: a first liquid crystal cell comprising a first region having a first photon retardation value and a second region having a second photon retardation value; a second liquid crystal cell comprising a first region having said first photon retardation value and a second region having said second photon retardation value; wherein each of the first and the second retardation values are defined by a first excitation source and a second excitation source communicating respectively with each of the first and the second regions of said first and the second cells.
 110. The optical filter of claim 109 wherein the first region of the first stage is substantially aligned with the first region of the second stage.
 111. The optical filter of claim 109 wherein the first cell and the second cell form one of a Lyot filter, an Evans split element filter, a Solc filter, or a hybrid filter.
 112. An optical filter comprising: a pixelized liquid crystal stage for filtering photons; a voltage source operably connected to the liquid crystal pixels; and means for controlling the amount of voltage applied by said voltage source to ones of said pixels; wherein the liquid crystal is substantially uniformly thick in the direction of photon travel therethrough.
 113. The optical filter of claim 112 wherein the amount of voltage applied to a first set of ones of said pixels is substantially the same.
 114. The optical filter of claim 112 wherein the amount of voltage applied to each of said pixels is substantially the same.
 115. An optical filter comprising: a columnized liquid crystal stage for filtering photons; a voltage source operably connected to the liquid crystal columns; and means for controlling the amount of voltage applied by said voltage source to ones of said columns; wherein the liquid crystal is substantially uniformly thick in the direction of photon travel therethrough.
 116. The optical filter of claim 115 wherein the amount of voltage applied to a first set of ones of said columns is substantially the same.
 117. The optical filter of claim 115 wherein the amount of voltage applied to each of said columns is substantially the same. 